Glass sheets called safety glass sheets are used as front windshield glass sheets of vehicles. Safety glass generally includes a reinforced glass (tempered glass) and a laminated glass.
Tempered glass sheets are produced by, for example, heating glass sheets up to about their softening point, and blowing air to both surfaces of the glass sheet to rapidly quench the glass sheet, thereby forming a compression stress layer on the surfaces of the glass sheet. Glass is generally resistant to compression. Therefore, a glass sheet having strength of several times that of a glass sheet which is not thus treated is obtained by the presence of such a compression stress layer.
On the other hand, a method of reinforcing a glass includes a chemically tempering method by a so-called ion exchange method, in addition to the above-described tempering.
As a chemically tempered glass suitable for windshields of automobiles, airplanes, etc., Japanese Patent Publication No. 47-1312 discloses lithium or sodium aluminosilicate glass sheets and a method for producing the same. This Publication describes on page 3, right column, lines 29 to 34 that an especially preferred glass composition comprises 2-6% Li.sub.2 O, 5-10% Na.sub.2 O, 15-25% Al.sub.2 O.sub.3 and 60-70% SiO.sub.2, in terms of % by weight of the respective oxide basis, with the sum of Li.sub.2 O, Na.sub.2 O, Al.sub.2 O.sub.3 and SiO.sub.2 being at least 95% by weight of the composition.
WO/94/08910 (PCT/FR93/01035) provides glass for windshields of aircraft, etc. A chemically tempered glass is shown therein as a countermeasure for flying objects (such as collision of flying birds). The chemically tempered glass has a composition comprising, in % by weight: 65.0-76.0% SiO.sub.2, 1.5-5.0% Al.sub.2 O.sub.3, 4.0-8.0% MgO, 0.0-4.5% CaO, 10.0-18.0% Na.sub.2 O, 1.0-7.5% K.sub.2 O, and 0.0-4.0% B.sub.2 O.sub.3.
On the other hand, laminated glasses comprising, for example, two glass sheets which are laminated through an interlayer are widely used as front windshield glass sheets of automobiles, etc. Those laminated glasses are produced by, for example, laminating two glass sheets through a PVB (polyvinyl butyral) film.
The laminated glass of this type is an untempered sheet in that a plain stock glass sheet is not tempered. Therefore, even if the laminated glass is broken in accidents, it is broken into large pieces, so that a driver's visual field is not prevented. Soda-lime glass sheets formed by a float method are widely used as the plain stock glass sheet for such a laminated glass.
JP-A-54-100415 discloses a windshield laminated glass having an inner sheet of a chemically tempered silicate glass, wherein a glass sheet having a thickness of 1.5 mm or less produced by a Fourcault process is chemically tempered and is used as the inner sheet of the windshield laminated glass (The term "JP-A" used herein means an "unexamined Japanese patent application"). The object of the invention disclosed in JP-A-54-100415 is to reduce the damage to the head of drivers and fellow passengers in low-speed collisions. Specifically, JP-A-54-100415 provides a laminated glass having a severity index (SI) of 1,000 or less. Although its glass composition is not specifically disclosed therein, it is presumed that a glass sheet of the laminated glass will be an easily-producible soda-lime glass sheet in view of the facts that the glass sheet is a silicate glass sheet produced by a Fourcault process and the laminated glass is for windshield.
Vehicles running at high speed, such as railroad vehicles or automobiles, often fly up small stones and others existing around railways and roads due to the wind pressure during their running. If the thus flung-up stones and others (hereinafter referred to as "flying stones") smash into the running vehicles, the surface of the window glass of the vehicles will be scratched, and breakage of the window glass may occur at the worst.
Therefore, it is required to prevent as much as possible the window glass of vehicles from being scratched by those flying stones or the like to such a severe degree that the thus-scratched window glass must be exchanged for new one, and also from being broken.
It is considered as a means for solving the problem to use a tempered glass having compression stress formed on its surface as the window glass for vehicles in order to protect it from flying stones.
However, use of the tempered glass sheet has the problem that, if damaged, it is broken into small pieces to thereby interfere with the visual field of drivers and passenger crew. For this reason, zone tempered glass sheets have heretofore been used as front windshield glass sheets of automobiles, in which the degree of tempering is decreased in the part in front of drivers.
However, in Japan, the front windshield glass of passenger cars must be made of a laminated glass as ruled in the law. Therefore, a single sheet of tempered glass cannot be used as the front windshield glass of automobiles. For other automobiles except passenger cars, it is desirable to use a laminated glass as the front windshield glass for safety purposes.
Some examples are known of using tempered glass sheets or zone tempered glass sheets to produce a laminated glass for vehicles.
Where a laminated glass comprising tempered glass sheets are used in vehicles, it is believed that such a laminated glass will be effective to protect the surface thereof from scratches by flying stones and others due to that compression stress is formed on the surface. However, if the laminated glass is broken, the laminated glass is entirely broken in view of the properties of the tempered glass. The entire breakage of the laminated glass used as the windshield shall interferes with the visual field of drivers and passenger crew, and shall even make passengers and fellow passengers have an extreme fear.
Even if a zone tempered glass sheet is used, when it is broken it also interferes with the visual field of drivers and passenger crew even though the visual field when broken is improved to some degree.
In other words, where tempered glass sheets (including zone tempered glass sheets) are laminated to form a laminated glass and such is used in vehicles, the resulting laminated glass is, when broken, entirely broken to thereby make passengers and fellow passengers have an extreme fear, and such an entire breakage interferes with the visual field of drivers and passenger crew. For these reasons, it is disadvantageous to use such tempered glass sheets as a countermeasure for flying stones.
On the other hand, glass can also be tempered by a chemical tempering method by an ion exchange method as described above.
Soda-lime glass sheets formed by a float method are widely used as glass sheets for vehicles. It is general in the chemical tempering method of such soda-lime glass sheets by an ion exchange method to exchange Na.sup.+ for K.sup.+, because the soda-lime glass sheets contain a lot of Na ions.
However, even if soda-lime glass sheets are chemically tempered, the surface compression layer formed thereon has only a thickness as small as at most several tens of .mu.m. Therefore, if chemically tempered soda-lime glass sheets are used in vehicles, the scratch resistance of such chemically tempered soda-lime glass sheet as the countermeasure of flying stones is merely equal to or is rather worse than that of a non-tempered plain stock glass sheet.
In addition, since the ionic radius of K.sup.+ is relatively large, the ion exchange of Na.sup.+ with K.sup.+ takes a relatively long time. Therefore, this is unfavorable from an industrial viewpoint. The ionic radius of Na.sup.+ is 0.099 nm, and that of K.sup.+ is 0.138 nm.
The windshield glass disclosed in JP-A-54-100415 is one having a chemically tempered silicate glass sheet as the inner sheet. Therefore, if this is used as it is, it does not act as a countermeasure for flying stones. Further, even if a soda-lime glass sheet chemically tempered is used as the outer sheet, if such is a glass sheet produced in a Fourcault process, the resulting laminated glass still does not effectively act as a countermeasure for flying stones for the same reasons as described above.
The glass composition disclosed in Japanese Patent Publication No. 47-1312 requires high temperature to melt and mold it, and it is difficult to produce high-quality glass from such a glass composition by a float method.
WO/94/08910 requires a long time of at least 72 hours or longer, and preferably 10 days or longer, for the ion exchange, and this is not a practical production method in view of cost.